MadSci Network: Astronomy

Re: Can refraction from Sunlight cause a hotspot behind the Earth?

Date: Mon Oct 5 19:50:20 1998
Posted By: Adams Douglas, Staff, R/D, Dicon Inc.
Area of science: Astronomy
ID: 906843605.As

Interesting question, Rod.

Sunlight does refract through Earth's atmosphere but it's filtered and 
diffused by the atmosphere's thickness and varying density. 

When you see a total lunar eclipse, you often see the moon turn blood 
red--this is from the sunlight that passes through the atmosphere around 
the edge of the Earth as seen from the moon. It's the same kind of reddened 
light you see during a sunrise or sunset. In fact, the Moon is red during a 
lunar eclipse from the light of all the sunsets and sunrises happening on 
Earth at once.

Another factor to consider is just how thin the atmosphere is with respect 
to the Earth. Although the "top" of the atmosphere is considered to be at 
about 75 miles, most of the the air is in the densest part within 20,000 
feet of the surface. This is why mountain climbers need extra oxygen. So 
the part of the atmosphere that refracts sunlight the most is so thin 
compared to the Earth that very little sunlight is getting refracted into 
the area behind the Earth, compared to the amount of sunlight blocked by 
the Earth itself.

Even if you traveled far enough behind the Earth to be beyond the 
umbra (the cone of shadow where direct sunlight is blocked 
completely) you'd still be in the penumbra where the Earth is 
partially obscuring the Sun. And the amount of sunlight being obscured by 
the Earth would always be bigger than the amount of sunlight passing 
through the atmosphere.

The best bet for Space Solar Power is to put your power satellite somewhere 
where it won't be eclipsed by the Earth or the Moon very often so you get 
as much raw, direct sunlight as possible.

Hope that helps you plan your mission.

-Adams Douglas
 Senior Developer
 Dicon Inc.

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